1. Field of the Invention
The present invention relates in general to optical processing systems, and in particular to an improved method and apparatus for modulating a light beam in an optical processing system.
2. Description of the Prior Art
Many optical processing systems utilize spatial light modulators to encode and/or filter information onto or from a light beam, typically a coherent laser beam. These spatial light modulators operate by modulating a light beam in two dimensions by changing the intensity and/or phase of the light wave in a controllable manner. Thus, they can be used to introduce spatial patterns onto a light beam. The commercially available spatial light modulators however tend to be expensive. As a consequence, many optical processing systems utilize liquid crystal video displays as an inexpensive and readily available alternative to spatial light modulators. They may be obtained from commercially-available color video projector units, such as the Crystal Image Video Projector unit Model No. E1020, manufactured by Epson. Such projector units have a reasonable optical quality insofar as they are spatially uniform. Furthermore, each liquid crystal display is individually electrically addressable. The displays are typically driven utilizing a specialized electronic drive circuit which is supplied with the projector unit. The drive circuit acts as an interface between an analog video signal and the two dimensional matrix of pixels which make up the liquid crystal video display at that particular pixel. The voltage across each pixel of the liquid crystal video display is proportional to the gray scale level at the corresponding location in the video display. Typically, the liquid crystal video displays are twisted-nematic liquid crystal devices, which means that the optical birefringent axes, caused by the anisotropy of the liquid crystal molecules, rotate throughout the thickness of the cell, in a manner similar to that depicted in FIG. 1A. The birefringence is altered or modulated by the application of the drive voltage (which is derived from the video signal) across a pixel. This change in the birefringence causes a change in the polarization state of any light beams which pass through the liquid crystal video display. If the liquid crystal video display is disposed between two polarizing optical instruments, the polarization modulation is converted to amplitude or phase modulation of the light beam. It is the amplitude and/or phase modulation of a light beam which allows for the processing of images. More specifically, the amplitude and/or phase modulation can be utilized for either encoding information onto the light beam, or for filtering or otherwise manipulating information on the light beam. Since the interaction between the light beam and the liquid crystal video display is fairly complicated, a simple amplitude-only modulation or phase-only modulation is not possible.
The liquid crystal video displays are merely one type of "complex spatial light modulator" which are utilized in optical processing systems. A complex spatial light modulator can be characterized as an optical processing device whose action on a light beam may be expressed as affecting the phase and amplitude (or alternately, the real and imaginary parts) of an incident light beam. Typically, spatial light modulators, such as the liquid crystal video display, are useful for optical processing of images as a function of position over the active area of the optical processing device. While liquid crystal displays may be electrically addressed on a pixel-by-pixel basis, other types of complex spatial light modulators are addressable as a continuous function of position. For purposes of the present application, the terms "complex spatial light modulator" and "spatial light modulator" will define all optical devices which affect the phase and amplitude of an incident light beam; however, the preferred embodiment discussed herein will specifically refer to a liquid crystal video display, which is merely one type of complex spatial light modulator.
The prior art devices used for generating and analyzing specific polarization states include one device developed by J. L. Pezzaniti and R. A. Chipman which permits the automated measurement of the Meuller matrix of a spatial light modulator. This device is described in detail in the following publications:
J. L. Pezzaniti and R. A. Chipman, "Phase-only Modulation Of A Twisted Nematic Liquid-Crystal TV By Use Of The Eigenpolarization States", Optics Letters vol. 18, pp. 1567-1569 (September 1993); and PA1 J. L. Pezzaniti, R. A. Chipman, and D. A. Gregory, "Polarization Characterization Of A LCTV With A Meuller Matrix Imaging Polarimeter", in Optical Pattern Recognition IV, D. Casasent, ed., Proc. SPIE vol. 1959, pp. 266-281 (1993). PA1 J. L. Pezzaniti, Meuller Matrix Imaging Polarimetry, PhD Thesis, University of Alabama in Huntsville (1993).
A major distinction of the present invention from the Pezzaniti and Chipman device is that the Pezzaniti and Chipman device requires gross physical motion (rotation) of polarizers and retarders in order to generate and analyze its set of polarization states. In contrast, the only physical motion in the present invention is the rearrangement of liquid crystal molecules. Another distinction is that the art of Pezzaniti has its preferred embodiment as a laboratory bench tool to characterize the polarization properties of spatial light modulators, as opposed to the present invention's major operational intent of providing immediate access to any of a plurality of different polarization behaviors. Accordingly, it is slower, more accurate, more stable, bulkier, heavier, more complicated, and more precise that the present invention.